Playing devils advocate here, what if he only used the SSL or the pellets he used in the mag were the same as the two options Brocock used and shot acceptably when you returned it? If it's just the magazine issue, what about a CARM? It's got to be easier than messing about with returns, or as you now know what they shoot with, how about trying a few tins of them?
https://www.carm-magazines.com/magaz...ir-rifles.aspx